The present invention relates to pressure measuring apparatus and more particularly to pressure measuring apparatus for use in cardiovascular direct fluid pressure measuring systems.
Direct blood pressure measurement in the cardiovascular system of a patient encounters a particular problem relating to the occurrence of bubbles in the measuring apparatus. Such apparatus typically includes flow lines and associated apparatus such as syringes, transducers, valves and the like. This equipment is normally made of highly transparent plastic materials which permit the observance of the formation and accumulation of air bubbles in fluids therein. Air bubbles may be formed by fluid flowing past structural features forming impediments in the fluid flow system. Such impediments may be in the form of sharp corners, or any feature, for that matter, which causes turbulence in the fluid flow to thereby entrain air bubbles in the fluid. Again, corners, pockets or the like, in the flow path through such equipment can be a likely place for bubbles to accumulate. Therefore, such equipment is designed to limit, to the extent possible, such structural features.
Systems for the direct measurement of blood pressure of a medical patient normally employ an arterial catheter inserted into an artery or vein and extending therein to the point of measurement. A fluid flow line connects the catheter to a manifold system. The manifold utilizes a syringe to inject a sterile transmitting fluid into the pressure measuring system. A transducer, also connected to the manifold, provides a means for converting the pressure state of the transmitting fluid into an electrical signal for providing intelligible data. If bubbles are formed and accumulate in any of the fluid flow portions of the above described system, serious problems may result which can adversely affect not only the accuracy of the pressure measurements but also the well being and perhaps the life of the patient being monitored.
The pressure measuring system described above utilizes a sterile transmitting fluid in the system which actually interfaces at one point directly with the blood flowing in the cardiovascular system of the patient. A variety of flow lines and interconnecting valves communicate this transmitting fluid with the transducer. This system, therefore, directly transmits the patient's blood pressure to the transmitting fluid which connects the pressure with the transducer. Such a system assumes that the transmitting fluid is non-compressible. However, if air bubbles exist in the transmitting fluid, this assumption is incorrect and consequently errors will occur in the data reading out of the transducer. A large bubble in the system can create errors, for example, in the range of 10 to 20%. The readout of such equipment is usually in a pressure-versus-time format, and such errors thus change the frequency response of the system, which in turn is reflected in the damping and distortion of a waveform being displayed on a visual monitoring device. Such errors are of sufficient nature to perhaps change the diagnosis of a patient's condition which may result in an erroneous disposition of his treatment.
Another aspect of the problems with air bubbles in such a system may result when a fluid is injected into the cardiovascular system. This may occur when the transmitting fluid of the pressure measuring system described above is injected by the syringe into the fluid flow system. Any air bubbles entrained in the fluid could be transmitted directly into the patient's cardiovascular system, giving rise to the danger of air embolism which may create turbulence and even disruption of blood flow. This, of course, can be a life threatening situation.
In order to avoid as much as possible the problems and dangers associated with air bubble entrainment in fluid flow systems of this sort, those parts of the systems which are subject to the occurrence of bubbles are manufactured from a transparent material. This is accomplished, for example, by using highly polished dies in plastic extruding or injection molding equipment. By rendering the surface as smooth as possible, the transparency of the materials used in manufacture is maximized. This in turn permits observation of the fluid for detecting bubbles, which may then be dissipated as by tapping or jarring the equipment. It has also been thought that by minimizing disturbances in the fluid flow path, the chance of creating turbulance is decreased. This then would lessen the chance of creating air bubbles.
The transducer used in such systems generally comprises a cylindrical housing having top and bottom chamber portions separated by a flexible membrane. A dome is arranged over the top of the membrane and has a cavity formed therein to form the top chamber. This top chamber is connected by means of ports in the dome walls to the fluid flow lines of the pressure measuring system. The bottom chamber houses a strain gage or the like which is positioned against the bottom surface of the membrane to detect movement of the membrane and provide an electrical signal indicative of such movement. The top chamber formed by the cavity serves as a fluid accumulator and to bring fluid into interface with the flexible membrane. Pressure fluctuations originating in a patient's cardiovascular system are transmitted by means of the transmitting fluid and the arterial catheter to the fluid flow line connecting with the manifold and transducer. Thus, the pressure pulses enter the cavity in the transducer dome where they flex the membrane which in turn produces an electrical signal for operating readout equipment.
The cavity within the transducer dome has been heretofore constructed with smooth surfaces, free of corners and irregularities, and generally forming a symmetrical chamber about a flexible membrane. However, it is now found that the fluid dynamics of the top chamber cavity in the dome structure described above may produce standing waves in response to pressure pulses. The fluid dynamics of the system relate to impedance matching between various components, and to the potential for setting up standing waves. The present invention is concerned with, among other things, the fluid impedance offered by the dome portion of a pressure transducer in cardiovascular direct pressure measuring systems. If the impedances of the input and output tubes are not matched to the impedance of the dome cavity at the points of entry of the tubes into the cavity, an incoming pressure pulse into the dome cavity will be reflected back toward the source, thus producing a standing wave within the cavity. Additionally, a portion of the wave energy incident to the cavity will be reflected rather than enter the cavity. Subsequent incoming pressure pulses will strike the standing wave and will either add or substract depending on the phase between the waves. A symmetrical cavity or chamber will develop and offer a resonate enclosure for standing waves. If standing waves are present in the cavity, the behavior of the membrane and strain gage may not truly reflect the pressure in the cardiovascular system to be measured; rather, false pressure readings may be observed, whether too high or too low.
It is therefore an object of the present invention to provide a new and improved fluid pressure measuring system with features for eliminating the presence of bubbles and standing waves in the system.